This invention relates to apparatus for protecting an electric power system wherein a remote electric quantity is reproduced by transmitting a frequency modulated carrier wave and the reproduced signal is used to protect the electric power system, and more particularly to apparatus for detecting and processing an abnormal condition of the transmitted signal.
To aid understanding of the invention a protective relaying system to which the invention is applicable will firstly be described with reference to FIG. 1 of the accompanying drawing. In the system shown in FIG. 1, for the purpose of protecting a transmission line TL between two spaced electric power stations or systems A and B, the secondary current of a current transformer CT associated with the transmission line TL is applied to a relay unit including a transmitting unit and a receiving unit. The level of the secondary current is converted in the relay unit and then sent to the other electric station after being subjected to a frequency modulation in the transmitting unit. Similarly, the frequency modulated signal from the other power station is received by the receiving unit of one station anddemodulated. The demodulated signal and the signal in one station are used to operate the relay unit, for causing it to act as a differential relay or a phase comparison relay, for example.
In such a relaying system it is necessary to correctly detect the abnormal conditions of the transmitted signals in order to prevent misoperation of the relaying system caused by the abnormal conditions. To this end, a frequency range supervising system has been used. More particularly, in the frequency modulation transmission system as the carrier frequency F.sub.o is varied by .+-. .DELTA.F the frequency of the received wave is in a range of from F.sub.o - .DELTA.F to F.sub.o + .DELTA.F. In this case, the frequency of the received wave is supervised to determine that signals having frequencies outside of this range are abnormal signals. However, the detection efficiency of this system is low. Even when the frequency is varied due to a noise, so long as the received frequency is included in this range any abnormal condition could not be detected. However, the demodulated waves are often abnormal. A remarkable example of this case is shown in FIG. 2 in which FIG. 2a shows a transmitted wave and FIG. 2b a received wave. In both FIGS. 2a and 2b the transmission delay time is omitted. In this example, F.sub.o = 1,800 Hz, and .DELTA.F = 600 Hz so that the normal frequency range is from 1,200 to 2,400 Hz. FIG. 2a shows a portion of a transmitted wave having a frequency of 1,200 Hz while FIG. 2b shows a corresponding portion of the received wave but the waveform of a portion near zero point is disturbed due to a noise and has a frequency of 2,400 Hz. By the frequency range supervising system, it is impossible to detect an abnormal condition, yet the received signal is abnormal because its waveform is greatly disturbed by the noise. Excluding such an extreme case, there are many cases in which even the variations in the normal frequency range cannot be neglected. FIGS. 3a and 3b show one example of such a case wherein the zero point of a signal having a frequency of 1,800 Hz (FIG. 3a) has been shifted by the noise so that the frequency of one period has decreased to 1,500 Hz whereas that of the next period has increased to 2,250 Hz as shown in FIG. 3b. In this case too, the abnormal condition is not detected yet an adverse effect would cause the same trouble as in the case shown in FIG. 2.